Mobility management is a challenging task in cellular communications systems. A factor having a crucial impact on the performance of mobility management schemes is the setting of mobility triggers, which is discussed in the article “Trends In Handover Design” by G. P. Pollini, published in IEEE Communications Magazine, March 1996 and in the article “Handoff in Cellular Systems” by N. D. Tripathi, J. H. Reed, and H. F. Vanlandingham, published in IEEE Personal Communications, December 1998.
There are a number of mobility triggers used in different communications systems, but the majority of systems consider two types of hysteresis as mobility triggers:                a hysteresis in signal quality; and        a time hysteresis (duration) during which the hysteresis condition related to the signal quality has to be satisfied.        
The optimal set of mobility trigger depends on a number of factors. Such factors are e.g. speed of the user equipment (UE), cell size and radio propagation environment (e.g. rural or urban area).
In the 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) system the major mobility triggers are termed HO (handover) hysteresis and Time To Trigger (TTT) for the connected mode and Qhyst and Treselection for the idle mode. In cellular communications systems according to other standards corresponding mobility triggers may be termed differently.
Studies, field measurements and experience from existing networks have shown that in order to have optimal mobility performance, there is a need to adapt the above mentioned mobility triggers as functions of UE speed, cell size and radio propagation environment. Consequently, there is a need for the UE or the network to detect that either the speed or the cell size or the radio propagation environment has changed and to update related mobility triggers accordingly. In this respect several methods of detecting UE speed either in the mobile or in the network can be found in the prior art. However existing solutions for speed detection have some drawbacks when trying to implement them in real cellular systems. Speed detection mechanisms which are based on measuring Doppler shift, are problematic due to the fact that Doppler shift is not always a good indicator of the speed in a wide range of mobility scenarios. This also increases power consumption in idle mode since the UE needs to be active for considerable duration to obtain reliable results. Other techniques for the detection of UE speed as a function of signal variations are estimated to be quite complex and quite power consuming. Thus, the implementation of these prior art methods for speed detection in commercial networks may be problematic, due to lack of accuracy and high complexity for most of them.
The 3GPP TS 36.304, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode (Release 8)”, version 8.6.0., published June 2009 suggests, in section 5.2.4.3, a simplified method for detecting a state of speed of a UE by counting the number of performed cell reselections or handovers within a given time window. However, it has been extensively argued that this method is vulnerable to ping-pong decisions and it requires some reaction time when conditions change. In addition, this method provides adaptation only in function of UE speed and it does not consider the other factors having an impact on the performance of mobility, such as cell size. Furthermore, the tuning of parameters so as to detect speed is quite complex. Also, speed estimation, which is based on the number of cell reselections performed, is to be done over a significantly longer duration to obtain reliable results compared with other speed estimation methods.
Within 3GPP LTE there has been discussed to use multiple sets of mobility trigger parameters configured by the network. More specifically, there has been discussed to use two sets of triggers: a ‘LONG’ set (long time hysteresis such as 2 seconds and small signal hysteresis such as 1 dB, or the like) and a ‘SHORT’ set (short time hysteresis such as 0.1 second and large signal hysteresis such as 4 dB, or the like) corresponding to low speed and high speed respectively. The scheme has been presented and its performance is assessed and compared with the performance of other schemes. The use of multiple sets of mobility triggers results in a higher number of cell changes out of which some of them cause ping-pong. This increases the signaling overhead in the system and the UE power consumption. Increase in the power consumption stems from the fact that the UE has to open its receiver for acquiring the necessary system information each time the UE changes the cell.